Tumors characteristically exhibit mutations that enhance cell proliferation and survival. Two well-recognized cell survival pathways are RAF--> MEK-->ERK and PI3-kinase--> Akt. Many inhibitors of these pathways are now in clinical trials or at earlier stages of development. However, early results suggest these inhibitors are more likely to suppress growth than kill tumor cells. Recent observations have demonstrated that such inhibitors may be more valuable when used in combination with more traditional cytotoxic anticancer agents. Specifically, it has been shown that a MEK inhibitor can dramatically enhance the rate of apoptosis induced by vinblastine in myeloid leukemia ML-1 cells and HL60 cells. The MEK inhibitor does not sensitize cells to paclitaxel. This difference is attributed to the ability of vinblastine, but not paclitaxel, to activate JNK. This has led to the hypothesis that vinblastine and other vinca alkaloids induce both an apoptotic and an anti-apoptotic response. The anti-apoptotic response involves rapid transcriptional activation of protective genes, which appear to be co-regulated by ERK and JNK. The rapid induction of apoptosis is also dependent upon JNK, but acts at the post-translational level. One goal of this proposal will be to identify the critical genes involved in the anti-apoptotic response in ML-1 cells, and to determine how the ERK and JNK signaling pathways coordinate the expression of these genes. Two candidate genes are the Bcl-2 family members Mcl-1 and Bfl-1 that are induced rapidly by vinblastine. The second goal will be to study other leukemia cell lines such as U937, which are insensitive to the MEK inhibitor but are sensitized to vinblastine by an inhibitor of PI3-kinase. Bcl-2 family members other than Mcl-1 are predicted to be targets of the PI3- kinase pathway. Two myeloma cell lines have also been identified that are acutely sensitized to vinblastine by inhibition of protein synthesis, but not by inhibition of either ERK or PI3-kinase. The third goal will be to establish the critical signaling pathways and downstream effector proteins in these myeloma models. These observations demonstrate that different leukemia's preferentially use different survival pathways and effector proteins. The ultimate goal of this research program will be to define which pathway and effector a specific leukemia uses, and then prescribe effective drug combinations that will be individualized for that patient.